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To Bleach or Not to Bleach, That Is Only Half the Question of Coral Reef Response to Global Warming
Volume 4, Number 3: 17 January 2001

The massive El Niņo of 1997-98, which pushed sea surface temperatures to some of their highest levels of the past century, created a great stir among students and aficionados of coral reefs.  It also raised a number of concerns among many indigenous peoples who depend upon reef ecosystems for their sustenance - either directly, via sea-life harvests, or indirectly, via tourist-generated income - as near-synchronous mass coral bleaching was observed to occur contemporaneously with the anomalous El Niņo-induced warming in several of the world's oceans.

Many environmentalists and media personalities equated this phenomenon with the beginning of what they said would be the unavoidable end of CO2-induced global warming, citing as evidence for their claims an impressive review of the subject by Hoegh-Guldberg (1999).  However, their doomsday perspective was challenged by the more optimistic assessment of Idso et al. (2000), who conducted an equally rigorous review the scientific literature and concluded that the predicted demise of the world's coral reefs was based on rather tenuous evidence and that much remained to be learned before the dire prognostication could be given any credence.

As the saga of the competing theories continues to evolve, the two ways of looking at the issue continue to be elaborated.  In a report in the 1 December 2000 issue of Geophysical Research Letters, for example, Lough (2000) provides additional support for the thesis of Hoegh-Guldberg.  In a study of sea surface temperatures recorded at 47 coral reef sites around the world, he shows that 1998 temperatures were the warmest of the past century, and that the 1998 product of elevated temperature and its duration was fully four times greater than previous high values recorded in 1983 and 1958.

In light of these observations, it is not surprising, Lough informs us, that "during 1997-98 there were unprecedented numbers of reports of bleaching on many of the world's coral reefs."  The implication that the bleaching events were caused by the warm-temperature anomalies, however, is challenged by Stone et al. (1999), who note "it is possible that mass coral reef bleaching has been occurring unobserved throughout the last centuries [when it was almost always much colder than it is now] but has only been noticed in recent decades."  These researchers also suggest that the idea that the recent bleaching event was fundamentally linked to global warming - which provides a rising background temperature upon which El Niņo warming spikes are superimposed - "should be viewed more as an untested hypothesis, supported so far only by circumstantial evidence."

Be that as it may, there is another aspect of the global-warming/coral-bleaching debate that may be even more important than whether elevated temperatures truly induce coral bleaching, which we are willing to concede does indeed happen in many instances.  This other concern has to do with what happens during and after bleaching events in terms of resistance and response to prolonged episodes of elevated water temperature; and it is this other half of the famed Shakespearian question we now address.

In a recent study of the effects of the 1982-83 El Niņo, Glynn et al. (2000) observed that warming-induced coral mortality was approximately 50% in Costa Rica, 75 to 85% in Panama, and 97% in the Galapagos Islands.  However, they subsequently found that the reefs in these locations returned to pre-El Niņo conditions in only a decade, and that bigger strides in reestablishment were made in the warmer of the intervening years, demonstrating not only the fact of reasonably rapid reef recovery, but also the adaptability of the recovering corals to higher temperatures.

In the case of the more recent 1997-98 El Niņo, which Normile (2000) credits as causing "the most extensive coral bleaching event ever seen," a similar remarkable reef recovery appears to be underway.  Reefs in the Indian and Pacific Oceans, in particular, he says, "seem to be recovering more quickly than expected," and he quotes Terry Done of the Australian Institute of Marine Science as saying that this rapid recovery "may indicate that reefs are more resilient than we had thought."  In fact, Normile reports that reefs that "looked 'like a graveyard' after the 1998 bleaching" were found as early as March of last year to contain "a surprising amount of new coral."

A similar observation was made by Mumby (1999) in reefs off Belize.  Immediately after corals there experienced what the scientist called "two of the most severe disturbances of recent decades: massive coral bleaching and Hurricane Mitch," which severely damaged 70 to 90% of adult coral colonies, only 25% of new coral recruits exhibited signs of bleaching.  And only one month after the damage was done, net bleaching-induced mortality of coral recruits was essentially nonexistent. "Surprisingly," the author writes, "coral bleaching alone had no measurable effect on either recruit density or community structure."

In much the same fashion, giant kelp plants located near Bahia Tortugas, Baja California, Mexico were essentially wiped out by the 1997-98 El Niņo.  From the fall of 1997 to the spring of 1998, in fact, not a single kelp could be found where extensive "forests" of these plants had grown previously.  Less than a year later, however, in July of 1998, according to Ladah et al. (1999), kelp plants were again observed to be growing in their old haunts.  As to how this amazing recovery of the large alga might have occurred, the authors suggest that "a microscopic stage that was not visible during dive surveys survived the stressful conditions of ENSO and caused the recruitment event, supporting the hypothesis that a bank of microscopic forms can survive conditions stressful to macroscopic algae."  And in this regard it is important to note that the symbionts of corals are also microscopic algae, and that they too may be thus preserved during times of intense thermal stress.

Another way by which corals may be able to survive prolonged periods of solar and thermal stresses that can cause severe bleaching was described by Salih et al. (2000) in the 14 December issue of Nature.  This most interesting paper, of which Hoegh-Guldberg is a co-author, describes several newly-discovered roles played by coral fluorescent pigments (FPs) in protecting reefs from intense solar radiation and high temperatures.  These pigments - whose "widespread abundance," according to the authors, "has not been previously reported" - dissipate excess energy at wavelengths of low photosynthetic activity when exposed to intense sunlight, as they also do by reflecting visible and infrared light.  In addition, the scientists determined that "FPs enhance the resistance to mass bleaching of corals during periods of heat stress," thus ensuring "enhanced survival of a broad range of corals" during episodes of anomalous warming, such as those that typically accompany El Niņo events.

In light of these important new discoveries about the abilities of corals and their symbionts to both resist and recover from the effects of severe solar and thermal stresses, and in consequence of the many real-world observations of rapid reef recoveries from coral bleaching events associated with the most massive El Niņo of the past century, it is fair to say that predictions of the demise of earth's coral reef ecosystems as a result of possible future global warming must be considered dubious at best and premature at the very least.  As knowledge continues to be acquired, it is becoming ever more evident that corals exhibit a tenacity for survival that is the complete antithesis of their legendary and oft-harped-upon fragility.

Dr. Craig D. Idso
Dr. Keith E. Idso
Vice President

Glynn, P.W., Colley, S.B., Ting, J.H., Mate, J.L. and Guzman, H.M.  2000.  Reef coral reproduction in the eastern Pacific: Costa Rica, Panama and Galapagos Islands (Ecuador). IV. Agariciidae, recruitment and recovery of Pavona varians and Pavona sp.a.  Marine Biology 136: 785-805.

Hoegh-Guldberg, O.  1999.  Climate change, coral bleaching and the future of the world's coral reefs.  Marine and Freshwater Research 50: 839-866.

Idso, S.B., Idso, C.D. and Idso, K.E.  2000.  CO2, global warming and coral reefs: Prospects for the future.  Technology 7S: 71-93.

Ladah, L.B., Zertuche-Gonzalez, J.A. and Hernandez-Carmona, G.  1999.  Giant kelp (Macrocystis pyrifera, Phaeophyceae) recruitment near its southern limit in Baja California after mass disappearance during ENSO 1997-1998.  Journal of Phycology 35: 1106-1112.

Lough, J.M.  2000.  1997-98: Unprecedented thermal stress to coral reefs?  Geophysical Research Letters 27: 3901-3904.

Mumby, P.J.  1999.  Bleaching and hurricane disturbances to populations of coral recruits in Belize.  Marine Ecology Progress Series 190: 27-35.

Normile, D.  2000.  Global Warming: Some coral bouncing back from El Niņo.  Science 288: 941-942.

Salih, A., Larkum, A., Cox, G., Kuhl, M. and Hoegh-Guldberg, O.  2000.  Fluorescent pigments in corals are photoprotective.  Nature 408: 850-853.

Stone, L., Huppert, A., Rajagopalan, B., Bhasin, H. and Loya, Y.  1999.  Mass coral reef bleaching: A recent outcome of increased El Niņo activity?  Ecology Letters 2: 325-330.